As is well known, lignin is a complex, high-molecular weight polymer occurring naturally in close association with cellulose in plants and trees. Lignin constitutes, on a dry weight basis, approximately 27% to 33% of the tree in softwoods and approximately 20% to 24% in hardwoods. In the paper-making industry, lignin is recovered as a by-product of the cellulose product by two principal pulping processes known as the sulfite process and the kraft process. In the sulfite pulping process, lignin is solubilized from the cellulosic portion of the wood pulp by direct sulfonation, while the kraft process is based on an alkaline degradation mechanism causing cleavage of .beta.-aryl ether linkages in the polymeric lignin which subsequently results in chemical functions of the phenolic and carboxylic type. Kraft lignin is isolated by acid precipitation from the black liquor of a kraft pulping process at a pH below the pKa of the phenolic groups.
The high degree of chemical activity which is characteristic of lignin salts permits the preparation of many novel and economical organic derivatives. Typical reactions which lignins can undergo are hydrogenation, halogenation, nitration, sulfonation, oxygenation, salt formation, etherification, and esterification. Lignin by-products variously have been employed in various chemical compositions as a surfactant, extender, dispersant, reinforcement, absorbent, binder, sequestering agent, emulsifier and emulsion stabilizer, and as a stabilizing and protective colloid. Lignosulfonate compounds, particularly sodium salts of lignosulfonates, have been employed as additives and dispersants in textile dyestuffs and printing pigments. Sodium salt sulfonated lignin by-products have been sold for many years under the trademark Indulin.RTM. by Westvaco Corporation of North Charleston, S.C.
Reduction in the pH of black liquor containing soluble lignin salts generally may be accomplished by introduction of carbon dioxide which converts the phenolic hydroxyl groups on the lignin molecule, which are in ionized form, into their free phenolic or acidic form. This conversion renders the lignin insoluble in the black liquor, and, as a result, it precipitates out. To precipitate the alkali lignin from the black liquor, the pH of the black liquor, initially around 13, is lowered to a pH of about 10.5 at which point lignin precipitation begins. Lignin obtained from the kraft process is not recovered as a sulfonated by-product, but is sulfonated, if desired, by reacting the material with a sulfur and oxygen-containing compound. Sulfonated lignins are understood to be those containing at least an effective amount of sulfonate groups to give water solubility in moderately acid and higher pH solutions.
One conventional process for sulfonating kraft lignins involves sulfomethylation of the alkali lignin by reacting the lignin with sodium sulfite and formaldehyde. Such a process is described in Adler, et al. U.S. Pat. No. 2,680,113. More recently, it has been proposed to sulfomethylate kraft process lignins in a two-step operation where the ionized phenol component of the lignin is methylolated at an alkaline pH by the addition of an aldehyde, the pH is then lowered to acid to precipitate the methylolated lignin and wash the precipitate to remove undesired inorganic salts, and the lignin thereafter sulfonated by addition of a salt, typically sodium, and also ammonium, of a sulfur and oxygen-containing compound. Such processes are described in commonly assigned U.S. Pat. No. 4,590,262 which issued May 20, 1986, and U.S. Pat. No. 4,642,336 which issued Feb. 10, 1987.
More recently, it has been proposed to produce sulfomethylated lignin amine salts of the lignin by-products of the black liquor residue of a kraft wood-pulping process. Such amine salts of lignosulfonates are particularly suited for use as additives in dyestuffs, pesticides, and other chemical compounds, and their production is disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 783,781, filed Oct. 3, 1985, U.S. Pat. No. 4,232,572. It has been found that sulfonated lignin salts containing a cation having a relatively low disassociation constant and pKa, such as the amine salts, have less tendency when used as a dispersant to cause azo dyestuff reduction. More particularly, lignosulfonate salts formed from reaction with an amine having a relatively low pKa, e.g., triethanolamine, diethanolamine, monoethanolamine, when used as dispersant in an azo dyestuff-containing dye system, causes less color reduction of the azo dyestuff than is the case in the use of the higher pKa lignin salts, such as sodium or ammonium.
As disclosed in the aforesaid co-pending application Ser. No. 783,781, amine salts of lignosulfonates may be prepared by (1) methylolating a lignin material, such as the lignin by-product of a kraft pulping process in alkaline liquid medium, (2) lowering the pH of the liquid to an acid pH to precipitate the methylolated lignin, (3) washing the precipitated methylolated lignin with water to remove inorganic salts and other impurities, and (4) thereafter reacting the purified methylolated lignin with an amine compound and a sulfur-oxygen-containing compound under moderately acid to neutral conditions to produce the lignosulfonate amine salt.
As set forth in co-pending, commonly assigned U.S. Patent application Ser. No. 859,384, U.S. Pat. No. 4,715,864 it has been more recently proposed to modify existing sulfonated lignin salts containing cations having relatively high disassociation constants and pKa's, e.g. sodium and ammonium, by addition of an amine compound having a lower disassociation constant and pKa, resulting in lignin salt compositions causing less color reduction of azo dyestuffs than unmodified higher pKa lignin salts. Decreased azo dyestuff reduction is believed to be brought about due to the capability of electrolytes to disassociate into two or more ions such that an equilibrium reaction takes place whereby the higher pKa lignosulfonate salt, e.g., sodium, and the lower pKa cation, e.g., amine, will re-arrange to equilibrate over time in a predictable association pattern. Thus, when the amine associates with the lignin to replace the sodium, lignin oxidation is inhibited, hence, so is azo dye reduction.
For economy of storage, shipment, and subsequent use, it is desirable to produce lignosulfonate salt additives in a dry form, rather than in a liquid slurry or solution. On a commercial basis, sodium salt lignosulfonates are produced in dry powder form by spray-drying the lignin salts after sulfonation. Conventional spray-drying equipment for carrying out such a spray-drying operation is disclosed in U.S. Pat. No. 2,081,909. In producing lignosulfonate amine salts, however, they have been found difficult to spray-dry, and heretofore have only been practically producable in a liquid medium or form. The difficulty of spray-drying amine salts is believed due to the presence of excess amine-containing compound which is necessary to raise the pH of the methylolated lignin slurry to the moderately acid to neutral level, e.g., above about 6, for the sulfonation reaction to be carried out on a commercially economical level. Triethanolamine, one typical amine employed in the production of the sulfomethylated lignin salts, is a weak base, and the amount required to raise the pH of the system to a point where sulfonation can proceed economically results in a lowering of the melting point of the reaction product and build-up of semi-molten material on the walls of the spray-dryer in subsequent spray-drying operations.
Various spray-drying approaches have been attempted to produce an effective, low-staining, non-reducing sulfonated lignin amine salt product. Adjustments of pH between 5.5 and 8.0 showed no noticable improvement in recovery. Additions of anti-static agent also met with no success. Although lowering dryer inlet temperatures from the normal operating spray-drying temperature of 260.degree. C. to 221.degree. C., increased recovery, the concommitant reduction in feed rate was not economically feasible.